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LTE and Wi-Fi. Better Together?

Introduction

Last month, T-Mobile announced plans to roll out LTE-U technology somewhere in the U.S. this spring. This means this controversial technology that has brewing since 2013 is finally going to become real for some of us, real soon. T-Mobile hasn't provided details about where in the U.S. LTE-U will be deployed, so we don't yet know who the lucky winners will be.

I have enough to do keeping up with the developements in consumer Wi-Fi, so haven't been tracking the LTE-U vs. Wi-Fi debate that closely. But now that it's about to poke its nose under the Wi-Fi tent, it's time to get a better handle on whether the technology is really going to play as nice with Wi-Fi as Qualcomm and other boosters of this new mobile network technology would have us believe.

What Are We Talking About?

Mobile carriers have had great success in getting us hooked on mobile video streaming. They've been so successful that they've oversold network capacity in many cases, which results in slower and less reliable connections and unhappy customers.

The approach to solving this problem until now has relied on moving data traffic to Wi-Fi networks, aka Wi-Fi offload. Many of us already use Wi-Fi offload whenever we can to avoid using expensive data minutes. Even folks with "unlimited" plans do this to avoid getting throttled down to a crawl when they hit their their oxymoronic "unlimited" data cap.

Various Wi-Fi offload methods are used depending on mobile provider, location and even internet service provider. But all suffer from the fact that the Wi-Fi networks that carriers are depending on to offload traffic aren't under their control. As a result, "automatic" offload methods often don't offer the seamless experience carriers would like. The net result is most of us handle switching to Wi-Fi manually.

The basic premise behind LTE-U and its related technologies is to put the use of unlicensed (Wi-Fi) 5 GHz spectrum under mobile carriers' control. This lets carriers make using the 5 GHz Wi-Fi band as invisible to mobile users as any other of the many licensed bands it already uses.

Technologies

LTE-U isn't the only technology looking to share unlicensed 5 GHz spectrum. The image below, taken from a Qualcomm infographic, shows the three major technologies in play. You'll often see references to LTE-U and LAA and more often, LTE-U / LAA. But, while similar, LTE-U and LAA are not the same.

Three ways mobile networks can use unlicensed spectrum

LTE (Long Term Evolution) - This is the base 4G technology developed by 3GPP (3rd Generation Partnership Project), the consortium that developed the GSM mobile network standard. Release 8 was the first LTE release.

LTE-U (LTE in Unlicensed spectrum) - This is the first pass at technology that uses the 5 GHz unlicensed band for mobile network traffic. It was initially developed by Qualcomm, based on the 3GPP LTE standard. The full specification was developed by the LTE-U Forum, an industry consortium formed by Verizon, Qualcomm and others in 2014. You can think of LTE-U as the time-to-market version of the more universal standard, LAA. LTE-U will be the first to be deployed in the US, Korea and India.

LAA (License Assisted Access) - This is the "global standard" version of the technology developed by the 3GPP, defined in 3GPP Release 13 (Q1 2016). No U.S. carriers have officially announced LAA deployments as yet. But it is thought LTE-U networks will eventually migrate to LAA.

Migration timeline

A key difference between LTE-U and LAA lies in the mechanism used to share bandwidth with 5 GHz Wi-Fi. LTE-U uses a mechanism called Carrier-Sensing Adaptive Transmission (CSAT), while LAA uses a mechanism called Listen Before Talk (LBT). The Qualcomm folks I spoke with for background in preparing this article tried patiently to explain the difference not only between CSAT and LBT, but also between the CSMA/CA mechanism Wi-Fi devices use to make sure they're not transmitting at the same time. This Qualcomm whitepaper may help you understand the difference better than it helped me.

My main takeaway is that LBT is required in Europe and other regions using ETSI standards, so that's what LAA must use. LBT is not required in the US, Korea and India, which is why LTE-U (with CSAT) can be used in those countries. Proponents of each say their technology will ensure fair bandwidth sharing with Wi-F and have run trials to prove it (here's one). But the proof will be in the verification.

LWA (LTE + Wi-Fi Link Aggregation) - An alternate approach to combining LTE and unlicensed networks that is also defined in 3GPP Release 13. 5 GHz band Wi-Fi is used to carry downlink traffic only. LTE carries all uplink, optionally downlink and manages both connections.

The graphic below taken from this Qualcomm presentation nicely illustrates LTE-U, LAA and LWA. LTE in licensed spectrum is combined with technologies in unlicensed spectrum in all three methods. The key difference is how the unlicensed spectrum is used, which will be explained further shortly.

Two ways to combine LTE and Wi-Fi

MulteFire - LTE technology operating solely in unlicensed spectrum. This is another mobile technology created by Qualcomm to enable LTE networks to operate in unlicensed spectrum. This doesn't seem close to deployment at this time. It could be a contender for use in the unlicensed 3.65 GHz  3.70 GHz band. It could also be used in the 5 GHz Wi-Fi band in campus type deployments. Qualcomm would like us to think of MulteFire as a more efficient way to use the 5 GHz band than 5 GHz Wi-Fi.

So What's the Problem?

As you read through this section, keep in mind that the 5 GHz frequencies / channels at the heart of the LTE-U/LAA vs. Wi-Fi debate are unlicensed spectrum. This means it is free for anyone to use without a license from a regulatory authority (the FCC in the U.S.) as long as certified equipement is used and technical requirements are followed.

Although the Wi-Fi industry has reaped the most benefit from the 2.4 and 5 GHz unlicensed bands, there is no law or regulation that says they are sole users of the spectrum. Many other devices including cordless phones, baby monitors, wireless cameras and even microwave ovens also share the space. Mobile network operators now simply want to use it too.

The nut of the problem is that LTE-U and LAA use standard 5 GHz Wi-Fi channels, i.e. 36-48 and 149-165 in the U.S., but not Wi-Fi protocols. Since Wi-Fi devices don't understand LTE-U / LAA protocols, they can't detect that LTE-U or LAA networks are in operation or properly contend for air time.

This puts the burden of fairly sharing 5 GHz Wi-Fi airtime solely on LTE-U / LAA devices. Wi-Fi devices can't participate in the sharing mechanism with LTE-based services because they can't understand LTE protocols. This is a situation similar to the introduction of 802.11n Wi-Fi; 11b and g devices didn't "speak" 11n. So 802.11n had to have mechanisms to accommodate "legacy" protocols, which it could detect.

Fair bandwidth sharing is the core issue that has been argued since Qualcomm first birthed LTE-U. It's probably how you first heard of LTE-U, and most likely in negative terms, as a technology out to "steal" 5 GHz Wi-Fi network bandwidth or kill the band altogether.

Qualcomm and unlicensed LTE proponents have said their technologies would not adversely affect Wi-Fi networks. And many trials and studies have been done to prove fair sharing. But with a global Wi-Fi market predicted to be $33.6B by 2020 vs. a 2016 global smartphone market alone worth $429B, you could see why Wi-Fi folks were not inclined to believe unlicensed LTE forces had their best interests in mind.

A deployed system transmitting in an unlicensed channel is operating fairly to Wi-Fi if the impact of that system on Wi-Fi users in the channel is no worse than the impact that would result from an additional Wi-Fi device or Wi-Fi network [emphasis mine] introduced into the channel supporting the same traffic load as the system.
A technology capable of being used in unlicensed channels is fair to Wi-Fi if its deployed implementations will always operate fairly as described above.

Note the Wi-Fi Alliance test plan is for LTE-U equipment only. 3GPP, the group developing the LAA specificiation is also developing an LAA coexistence verification plan.

Use of the Wi-Fi Alliance Coexistence Test Plan, however, is completely voluntary. T-Mobile and Verizon have publicly stated they will use the plan before deploying any LTE-U equipment. But I could find no similar statements from AT&T, Sprint or other U.S. carriers.